CRISPR-Cas9
A programmable genome-editing tool derived from a bacterial immune system that makes targeted DNA changes possible.
CRISPR-Cas9 is a technology for editing the DNA of living cells. It was adapted from an adaptive immune system found in bacteria and archaea: when a bacterium survives an attack by a virus, it stores a fragment of the viral genome in its own genome in a region called CRISPR (clustered regularly interspaced short palindromic repeats). On reinfection, the bacterium produces RNA copies of the stored fragments that guide the Cas9 nuclease to matching viral DNA, which Cas9 then cuts, disabling the virus.
The system is programmable in the laboratory. A researcher designs a short guide RNA whose sequence matches a target site in the genome of interest. The guide RNA forms a complex with the Cas9 protein, which scans the genome and binds wherever the guide sequence matches. For cleavage, the target must also be adjacent to a short motif called the PAM (protospacer adjacent motif, typically NGG for the commonly used Cas9 from Streptococcus pyogenes). The double-strand break is made at the target site.
The cell repairs the break using its own machinery, and the repair pathway determines the editing outcome. Non-homologous end joining (NHEJ) is error-prone and typically introduces small insertions or deletions that disrupt the gene — useful for creating gene knockouts. Homology-directed repair (HDR) uses a provided DNA template to copy in a precise change — useful for correcting mutations or inserting sequences. A variant of Cas9 that cuts only one strand, or a catalytically dead Cas9 fused to other enzymes, enables subtler operations such as base editing, gene activation, or fluorescent labeling of specific loci.
The technology's power lies in its simplicity: a new target requires only a new guide RNA, so experiments that once took months can be done in weeks. Applications include genetically modified crops with improved traits, disease models in mice and other organisms, and experimental human therapies — for example, engineered immune cells for cancer and clinical trials targeting sickle-cell disease. Because the edits are inherited, the technology also raises profound ethical questions, particularly about germline editing, where changes pass to future generations; many countries restrict or prohibit germline modification. Technical limitations include off-target cuts at similar sequences and the challenge of delivering Cas9 efficiently to the right cells in living organisms.
Tags
genetics genome editing molecular biology